1. Field of the Invention
The present invention relates to a focal point detecting apparatus for use in an electron microscope, an optical inspecting apparatus, a laser processing apparatus, etc. for detecting whether or not the focal point of an objective lens is positioned on a surface of a target object to be inspected.
2. Description of the Related Art
Heretofore, there has been proposed a focal point detecting apparatus employing a so-called "skew method" in which an optical beam for the focal point detection is entered from a position which is not on the optical axis of the objective lens in the optical system to the objective lens. In this method, a two-split sensor is positioned on the focal point of a condenser lens and a focus signal is obtained from a difference between two output signals generated from two light receiving elements constituting the two-split sensor.
An optical system in the conventional focal point detecting apparatus employing the skew method will be described in detail with reference to FIG. 1. As shown in the drawing, an objective lens 1 and a condenser lens 5 have an optical axis in common. A semitrasparent mirror 3 (or a polarization mirror) is arranged between objective lens 1 and condenser lens 5, and a two-split sensor 6 is arranged at the focal point of condenser lens 5.
An optical beam 2 for detecting the focal point from a position deviant from the optical axis la of objective lens 1 enters semitrasparent mirror 3 (or polarization mirror). Optical beam 2 reflected from semitrasparent mirror 3 passes through objective lens 1 to be converged on a surface 4 of a target object to be inspected. Then, optical beam 2 reflected from surface 4 of the target object enters again objective lens 1 and passes through semitrasparent mirror 3 to be converged on two-split sensor 6 by condenser lens 5
FIGS. 2A to 2C show the converged states of the optical beam on two-split sensor 6. A deviation of a beam 7 returning to the two-split sensor 6 on the side of a light receiving element 6A of the two-split sensor as shown in FIG. 2A, shows that surface 4 of the target object is positioned farther away from the focal point of objective lens 1.
The positioning of beam 7 returning to two-split sensor 6 equally on both of light receiving element 6A and a light receiving element 6B as shown in FIG. 2B, shows that surface 4 of the target object is positioned on the focal point of objective lens 1.
Further, a deviation of beam 7 returning to two-split sensor 6 on the side of light receiving element 6B of the two-split sensor as shown in FIG. 2C, shows that surface 4 of the target object is positioned closer to objective lens 1 than the focal point of objective lens 1.
Thus, the calculation of the difference in the output between the light receiving elements 6A and 6B of two-split sensor 6 makes it possible to detect surface 4 of the target object relative to the focal point of objective lens 1.
To this end, a signal processing circuit 8 is provided to compare the output signals from the light receiving elements 6A and 6B. Signal processing circuit 8 carries out F=a-b, where a and b represent output signals from the light receiving elements 6A and 6B, respectively.
FIG. 3 is a graph showing a variation in an output signal F from signal processing circuit 8 as surface 4 of the target object is moved along the direction of the optical axis of objective lens 1. In the graph of FIG. 3, the abscissa denotes a position of objective lens 1 relative to the focal point of objective lens 1. The larger the position from the origin becomes in the negative direction, the farther the surface 4 of the target object is positioned from the focal point of the objective lens 1. On the other hand, the larger the position from the origin becomes in the positive direction, the closer surface 4 of the target object is positioned to objective lens 1 than the focal point of objective lens 1. A curve 9 in FIG. 3 represents a difference signal F, which is a focus signal at the time when the light amount of return beam 7 is uniformly distributed about the optical axis of the return beam. As seen from FIG. 3, when the focal point of objective lens 1 deviates from surface 4 of the target object, objective lens 1 is moved along the direction of the optical axis such that the value of the focus signal F becomes zero, thus constantly making the focal point of objective lens 1 coincide with surface 4 of the target object.
The aforementioned conventional apparatus does not pose a serious problem when the light amount distribution of the return beam to two-split sensor 6 is substantially uniform about the optical axis. However, the light amount distribution of the return beam to two-split sensor 6 can not be uniform about the optical axis in the case of the surface of a target object having a large local difference in reflectance due to a wiring pattern depicted with chromium on a glass substrate like a semiconductor mask, or the surface of the target object having a difference in reflectance due to asperities thereof like a semiconductor wafer coated with resist, exposed, and then developed. Consequently, a desired focus signal cannot be obtained, causing a misoperation when the focus of the objective lens is adjusted on the surface of the target object.